Fluid diffusion resistant glass-encased optical sensor

ABSTRACT

A fluid diffusion resistant tube-encased fiber grating pressure sensor includes an optical fiber  10  having a Bragg grating  12  impressed therein which is encased within a sensing element, such as a glass capillary shell  20 . A fluid blocking coating  30  is disposed on the outside surface of the capillary shell to prevent the diffusion of fluids, such as water molecules from diffusing into the shell. The fluid diffusion resistant fiber optic sensor reduces errors caused by the diffusion of water into the shell when the sensor is exposed to harsh conditions.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/494,417, filed Jan. 31, 2000, now U.S. Pat. No. 6,626,043, which isincorporated herein by reference and to which priority is claimed under35 U.S.C. § 120.

This application is also related to: U.S. patent application Ser. No.09/399,404, filed Sep. 20, 1999, which is a continuation-in-part of U.S.patent application Ser. No. 09/205,944, filed Dec. 4, 1998, Ser. No.09/455,865, entitled “Tube-Encased Fiber Grating,” Ser. No. 09/455,866,entitled “Strain-Isolated Bragg Grating Temperature Sensor,” U.S. Pat.No. 6,229,827, entitled “Compression-Tuned Bragg Grating and Laser,”Ser. No. 09/456,113, entitled “Pressure Isolated Bragg GratingTemperature Sensor,” U.S. Pat. No. 6,278,811, entitled “Fiber OpticBragg Grating Pressure Sensor,” Ser. No. 09/455,868, entitled “LargeDiameter Optical Waveguide, Grating, and Laser,” and U.S. Pat. No.6,298,184, entitled “Method and Apparatus For Forming A Tube-EncasedBragg Grating,” filed Dec. 4, 1998. All of the aforementionedapplications and/or patents contain subject matter related to thatdisclosed herein.

TECHNICAL FIELD

This invention relates to tube encased fiber optic pressure sensors, andmore particularly to fluid ingression protection mechanisms for atube-encased fiber grating pressure sensor.

BACKGROUND ART

Sensors for the measurement of various physical parameters such aspressure and temperature often rely on the transmission of strain froman elastic structure (e.g., a diaphragm, bellows, etc.) to a sensingelement. In a fiber optic pressure sensor, the sensing element mayencased within a glass tube or housing comprised substantially of glass.One example of a fiber optic based sensor is that described in U.S.patent application Ser. No. 09/455,867, entitled “Bragg Grating PressureSensor,” filed Dec. 6, 1999, which is incorporated herein by referencein its entirety.

The use of fiber optic based devices is widespread in thetelecommunications industry, wherein the impervious nature of the glassprovides adequate protection given the relatively mild workingenvironments. A relatively recently known use of fiber optic pressuresensors is in an oil well to measure temperature and pressure at variouslocations along the length of the well bore. The sensors are typicallydeployed in metal housings in the wellbore and are attached on theoutside of the casing. Such sensors may often be subjected to extremelyharsh environments, such as temperatures up to 200 degrees C. andpressures up to 20 kpsi. These sensors are exceptionally sensitive andare capable of measuring various parameters, such as temperature andpressure, with extreme accuracy. However, the sensitivity and accuracyof fiber optic sensors creates problems when such sensors are used in aharsh environment. Known problems include poor signal to noise ratios,wavelength drift, wavelength shifts, optical losses, hysteresis andmechanical reliability issues. It is the realization of the theseproblems and the discovery of the causes that will advance the state ofthe art in fiber optic based well bore monitoring systems. One suchknown problem is “creep” of the sensor over time. It has been discoveredthat the attachment of the sensing element to the elastic structure canbe a large source of error if the attachment is not highly stable. Inthe case of sensors that measure static or very slowly changingparameters, the long-term stability of the attachment to the structureis extremely important. A major source of such long-term sensorinstability is creep, i.e., a change in strain on the sensing elementeven with no change in applied load on the elastic structure, whichresults in a DC shift or drift error in the sensor signal. Varioustechniques now exist for attaching the fiber to the structure tominimize creep, such as adhesives, bonds, epoxy, cements and/or solders.In addition, the sensors are subject to fluids containing hydrocarbons,water, and gases that can have deleterious effects on the accuracy ofthe sensors. For instance, it has been discovered that the performanceof wellbore deployed fiber optic sensors is adversely affected byexposure to hydrogen, which causes irreversible loss along the fiber'slength. Further, when the fiber optic sensors include Bragg gratings,exposure to hydrogen causes a shift in the index of the grating thatseverely lessens the accuracy of the sensor. Increased pressure andtemperature of the hydrogen increases the rate at which the fiber opticcables and sensors degrade.

It has also been discovered that certain side-hole fiber optic pressuresensors and eccentric core optical fiber sensors experience deleteriouseffects, such as those described above, when exposed to water at hightemperatures and pressures. The adverse effects are presumed to becaused by thin swollen surface layers that lay in close proximity to thesensitive fiber optic core. The observed shifts and changes are presumedto be due to the ingress of water molecules and the subsequent directexpansion of the silica that makes up the fiber itself. In oneparticular instance, the fibers had a core center-to-surface separationdistance of only 10 μm.

However, as discussed hereinbefore, many other problems and errorsassociated with fiber optic sensors for use in harsh environments stillexist. There is a need to discover the sources of these problems anderrors and to discover solutions thereto to advance the state of the artin fiber optic sensor use.

SUMMARY OF THE INVENTION

Objects of the present invention include a fiber optic pressure sensorwith fluid blocking provisions for use in a harsh environment.

According to the present invention, a fluid blocking fiber opticpressure sensor comprises an optical fiber having at least one pressurereflective element embedded therein, wherein the pressure reflectiveelement has a pressure reflection wavelength; a sensing element havingthe optical fiber and the reflective element encased therein, thesensing element being fused to at least a portion of the fiber and beingstrained due to a change in external pressure whereby the strain causesa change in the pressure reflection wavelength indicative of the changein pressure; and a fluid blocking coating disposed on the externalsurface of the sensing element.

According further to the present invention, the sensing elementcomprises a tube and the fluid blocking coating comprises at least onelayer. The fluid blocking coating comprises a fluid blocking material ofgold, chrome, silver, carbon, silicon nitride, or other similar materialcapable of preventing the diffusion of water molecules into to thesensing element. Alternatively, the coating comprises a first layercomprised of chrome disposed on the outside surface of the sensingelement and a second layer comprised of gold disposed on the firstlayer. In one embodiment, the first layer has a thickness of about 250 Åand the second layer has a thickness of about 20,000 Å.

The present invention also provides a fluid blocking fiber opticpressure sensor having a fiber grating encased in and fused to at leasta portion of a sensing element, such as a capillary tube, which iselastically deformable when subject to applied pressure. The inventionsubstantially eliminates' drift, and other problems, associated withwater or other fluid absorption into the tube. The tube may be made of aglass material for encasing a glass fiber. The invention provides lowhysteresis. Furthermore, one or more gratings, fiber lasers, or aplurality of fibers may be encased in the coated tube. The grating(s) orreflective elements are “encased” in the tube by having the tube fusedto the fiber at the grating area and/or on opposite sides of the gratingarea adjacent to or at a predetermined distance from the grating. Thegrating(s) or laser(s) may be fused within the tube, partially withinthe tube, or to the outer surface of the tube. The invention may be usedas an individual (single point) sensor or as a plurality of distributedmultiplexed (multi-point) sensors. Also, the invention may be afeed-through design or a non-feed-through design. The tube may havealternative geometries, e.g., a dogbone shape, that provides enhancedforce to wavelength shift sensitivity and which is easily scalable forthe desired sensitivity.

The invention may be used in harsh environments (i.e., environmentshaving high temperatures and/or pressures), such as in oil and/or gaswells, engines, combustion chambers, etc. In one embodiment, theinvention may be an all glass sensor capable of operating at highpressures (>15 kpsi) and high temperatures (>150° C.). The inventionwill also work equally well in other applications regardless of the typeof environment.

The foregoing and other objects, features, and advantages of the presentinvention will become more apparent in light of the following detaileddescription of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a fluid blocking tube-encased fiber opticsensor in accordance with an embodiment of the present invention;

FIG. 2 is a graphical representation of the performance of a prior arttubc-encased fiber grating sensor;

FIG. 3 is a graphical representation of the performance of atube-encased fiber grating sensor in accordance with an embodiment ofthe present invention; and

FIG. 4 is a graphical representation of the performance of analternative embodiment of a tube-encased fiber grating sensor inaccordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a tube-encased fiber Bragg grating transducer 1comprises a known optical waveguide 10, e.g., a standardtelecommunication single mode optical fiber, having a pressure Bragggrating 12 and a temperature Bragg grating 13 impressed (or embedded orimprinted) in the fiber 10. The fiber 10 is encased within shell 20 suchas is described in copending U.S. patent application Ser. Nos.09/455,867, entitled “Bragg Grating Pressure Sensor,” filed Dec. 6,1999, and Ser. No. 09/455,865, entitled “Tube-Encased Fiber Grating,”also filed Dec. 6, 1999, both of which are hereby incorporated byreference in their entirety.

In one embodiment, the transducer element 1 is constructed by fusing abare photosensitive fiber 10 in a fused silica capillary tube 15, whichfunctions as a piston as will be described herein below. In theembodiment shown in FIG. 1, Bragg grating 12 is approximately 5 mm inlength (although other lengths are possible) and is disposed in the“dogbone” region 16 of capillary tube 15, and Bragg grating 13 iscomprised of a different reflective wavelength and is disposed in athicker piston portion of the capillary tube 15 adjacent to the“dogbone” region 16. Grating 12 is used for interrogation of pressure(hereinafter pressure grating 12), while grating 13 is used todifferentially remove temperature effects that are common to allgratings (hereinafter temperature grating 13). A cylindrical,pressure-tight shell 20 is fused to the capillary 15, such that thegratings 12, 13 and dogbone region 16 are sealed inside thereby. Forsingle-ended transducers (not shown), an angle is polished on the end ofthe glass package opposite the fiber exit point to minimize backreflections and their detrimental effects to Bragg gratinginterrogation.

The mechanical principles of operation of transducer 1 are based on theelastic response of the shell 20 to an external pressure fieldrepresented by P. The sealed shell behaves like a thick walled pressurevessel. In one embodiment, the outside diameter of shell 20 isapproximately 6 mm although other lengths are possible and otherembodiments include shells that are integral with the sensor asdescribed hereinbelow. The shell 20 isolates the grating portions of thefiber and protects them from the harsh environment in which thetransducer is placed. An axial dimensional change of the shell 20,represented by LI, decreases in response to an end wall pressure 26 and(to a smaller extent) increases in response to a radial pressure 28 dueto radial Poisson's effects.

The dogbone region 16 acts like a relatively flexible tie-rod withintransducer 1, and senses the end wall axial displacement in response tothe pressure P. The reduced diameter (and hence stiffness) of thedogbone region 16 causes the majority of the axial displacement of theshell to be concentrated across this short region, enhancing the strainresponse of the Bragg grating 12 written within the fiber 10 topressure. The temperature grating 13 in the piston portion of capillarytube 15 necessarily exhibits an undesired response to pressure, thoughto a smaller degree, because of the larger cross-sectional area of thepiston region in relation to the dogbone region 16. This creates a lowernet response to pressure for the temperature grating 13, which isnecessary to differentially cancel the significant effects oftemperature on Bragg wavelength reflection.

Still referring to FIG. 1, the shell 20 (among other structures) isprovided with a layer 30 in accordance with the present invention toprovide a barrier to ingression of water, among other fluids, to theshell 20, the capillary tube 15, and gratings 12, 13 as will bedescribed more fully herein after. The fiber 10 has an outer diameter ofabout 125 microns and comprises silica glass (SiO₂) having theappropriate dopants, as is known, to allow light 14 to propagate alongthe fiber 10. The gratings 12, 13 are similar to those described in U.S.Pat. Nos. 4,725,110 and 4,807,950, entitled “Method for ImpressingGratings Within Fiber Optics,” to Glenn et al, and U.S. Pat. No.5,388,173, entitled “Method and Apparatus for Forming Aperiodic Gratingsin Optical Fibers,” to Glenn, which are hereby incorporated by referenceto the extent necessary to understand the present invention. However,any wavelength-tunable grating or reflective element embedded, etched,imprinted, or otherwise formed in the fiber 10 may be used if desired,or may comprise a Fabry-Perot type device. As used herein, the term“grating” means any of such reflective elements. Further, the reflectiveelement (or grating) 12, 13 may be used in conjunction with thereflection and/or transmission of light. Still further, the presentinvention encompasses embodiments wherein any number of gratings aredisposed within a glass, or substantially all glass, shell 20. Inaddition, the present invention includes any device wherein opticalfibers are used to measure the strain on a glass housing or shell andwhere the fiber is connected to the shell by an adhesive such as epoxyor other methods of attachment.

Other materials and dimensions for the optical fiber or waveguide 10 maybe used if desired. For example, the fiber 10 may be made of any glass,such as silica or phosphate glass, may be made of glass and plastic, orplastic alone, or other materials used for making optical fibers. Forhigh temperature applications, optical fiber made of a glass material isdesirable. Also, the fiber 10 may have an outer diameter of 80 micronsor other suitable diameters. Further, instead of an optical fiber, anyoptical waveguide may be used, such as a multi-mode, birefringent,polarization maintaining, polarizing, multi-core, or multi-claddingoptical waveguide, or a flat or planar waveguide (where the waveguide isrectangular shaped), or other waveguides. As used herein the term“fiber” includes the above-described waveguides.

The shell 20 and capillary tube 15 are made of a glass material, such asnatural or synthetic quartz, fused silica, silica (SiO₂), Pyrex® byCorning (boro silicate), Vycor® by Corning (about 95% silica and 5%other constituents such as Boron Oxide), or other glasses. The capillarytube 15 should be made of a material such that it (or the inner surfaceof the bore hole in the tube 15) can be fused to (i.e., create amolecular bond with, or melt together with) the outer surface (orcladding) of the optical fiber 10 such that the interface surfacebetween the inner diameter of the capillary tube 15 and the outerdiameter of the fiber 10 become substantially eliminated (i.e., theinner diameter of the capillary tube 15 cannot be distinguished from andbecomes part of the cladding of the fiber 10).

It has been discovered by the Applicant that the tube-encased fiberBragg gratings of the prior art exhibited significant drifts whendisposed in harsh environments in the presence of fluids. It wasdiscovered upon further investigation that the relatively imperviousnature of glass is severely degraded by elevated temperatures andpressures. Such drifting was discovered during the testing of fiberoptic based sensors as described in the above referenced copendingapplications. In accordance with such testing otherwise stable sensorswere immersed in a bath of silicone based oil at constant elevatedpressures and temperatures. Silicone oil was used because it was thoughtto be a stable fluid for transferring pressure to the transducer withoutcontamination. After a relatively short period of about one week at 170°C. degrees and using an atmospheric pressure sensor, both thetemperature and pressure gratings exhibited significant and rapid shiftsin wavelength as best shown in FIG. 2. FIG. 2 represents a fairlystandard plot for stability testing of fiber optic sensors wherein trace31 represents the relative reflected wavelength of pressure grating 12and trace 32 represents the relative reflected wavelength of temperaturegrating 13 over a 55-day testing period. In other words, traces 31, 32are plotted relative to the actual wavelengths and represent an offsetmeasurement therefrom. The relative offset of the reflected wavelengthof pressure grating 12 (trace 31) shows a fairly stable averagereflected relative wavelength of about 1.0013 nanometers for the firstfive days of the test. Between day five and day twenty-eight there wasobserved a significant shift of about 7.8 pico meters to a relativelevel of 1.0091 nanometers, at which point it seemed to stabilize for aperiod of about twenty-seven days (conclusion of the test). Similarly,but to a lesser extent, the relative offset of the reflected wavelengthof temperature grating 13 (trace 32) shows a fairly stable averagereflected relative wavelength of about 1.0035 nanometers for the firstfive days of the test and then shows a significant shift of about 1.5pico meters to a relative level of 1.005 nanometers where it to seemedto stabilize. Therefore, had the transducer been installed in anenvironment for monitoring the temperature and pressure of an oil wellhaving fluctuating conditions, the drift would have made accuratedetermination of the actual conditions impossible. For instance, for a 0to 15,000 psi operational range sensor, having a sensitivity of 0.3846pm/psi (or 2.6 psi/pm), this drift would translate into a 20.28 psierror.

In accordance with the present invention, it was discovered that traceamounts of water in the silicone oil were accountable for the driftsshown in FIG. 2. As discussed above, prior art fiber optic based sensorsexhibited drift due to the expansion of layers of the glass close to thefiber. Where glass tube encased sensors were used, the glass shell wasthought to be an adequate barrier, both in its composition and proximityto the gratings, to environmental influences on the accuracy of thesensors. However, through testing it was determined that at elevatedtemperatures and pressures the glass shell 20 (FIG. 1) absorbedsignificant amounts of water and caused the shell to expand therebycausing a wavelength shift in the gratings 12, 13 at constant pressureand temperature conditions. The expansion of the shell 20 as a result ofwater ingress has a greater influence on pressure grating 12 in thedogbone region 16 because of the concentration of the axial displacementacross the reduced cross section as described herein above and as shownby trace 31 in FIG. 2 when compared to trace 32.

However, applying a barrier layer 30 as shown in FIG. 1 eliminates thecause of the error associated with drift. Barrier layer 30, when appliedto the outside surfaces of transducer 1, eliminates the ingress ofwater, or other similar fluids, into shell 20 and thereby precludesexpansion of the shell and the drift caused thereby. Although shown ascoating optical fiber 10, capillary tube 15, and shell 20, embodimentsof the present invention encompass the coating of only shell 20 withlayer 30 to the extent necessary to preclude fluid ingress into theshell.

Layer 30 may comprise any material, or combination of materials, capableof preventing the diffusion of water molecules into shell 20. However,depending on the particular environment in which transducer 1 will beused, it may be critical to the operation of the transducer that layer30 not cause significant mechanical effects (including hysteresis) thatcould adversely affect the ability of the shell to react to pressurechanges. For instance, if the characteristics of layer 30 were such thatthe stiffness of shell 20 was significantly increased, the sensitivityand/or repeatability of the transducer may be unacceptably diminished.Other mechanical effects of the coating layer 30 which could havedeleterious effects on the operation of the transducer include coatingcreep, coating integrity, strain capability, etc. Both the materialchoice and thickness of layer 30 may contribute to these mechanicaleffects. Several materials have been considered based on their abilityto block water molecules, to adhere to the glass shell 20, and to limitthe amount of adverse mechanical effects. Among the materials consideredsatisfactory are chrome, gold, silver, carbon, and silicon nitride.However, other similar materials and combinations of materials arecontemplated by the present invention.

One embodiment of transducer 1 (FIG. 1) includes a coating 30 comprisedof a combination of a first layer of chrome and a second layer of gold.The coatings may be applied to shell 20 using a standard sputteringprocess as will be more fully described herein below, but the presentinvention should be understood to encompass any known method of coatingthe shell. In this particular embodiment, the chrome layer was appliedin a uniform manner to achieve a thickness of about 250 Å, and then asecond layer was applied in a uniform manner to achieve a gold layer ofabout 20,000 Å. Other satisfactory embodiments that have been testedhave gold layers as thin as 500 Å.

Coating layer 30 according to this embodiment is effective at reducingthe drift exhibited by the prior art as best shown with reference toFIG. 3. As in FIG. 2, trace 31 represents the relative reflectedwavelength of pressure grating 12 and trace 32 represents the relativereflected wavelength of temperature grating 13 over about a 46-daytesting period in conditions substantially identical to those describedwith respect to FIG. 2. The relative offset of the reflected wavelengthof pressure grating 12 (trace 31) shows a fairly stable averagereflected relative wavelength of about 1.008 nanometers for the firstten days of the test. Thereafter, between day ten and day twelve, asmall, but noticeable, shift of approximately 1.0 picometer is observedto a relative level of 1.009 nanometers where the sensor remained stableto the conclusion of the test. Coating 30 of this embodiment thusrepresents an improvement which is six times as effective at blockingwater, and its deleterious effects, when compared to the prior art. Inaddition, the reflected wavelength of temperature grating 13 (trace. 3²) shows an almost imperceptible change over the same time period. Inaddition, testing was performed on this embodiment to quantify themechanical effects of coating 30 on the sensor and to validate thatthere were acceptable levels of creep or hysteresis caused by thecoating.

In an alternative embodiment, coating layer 30 of transducer 1 iscomprised of a layer of carbon applied to glass shell 20, which againcan be achieved using a standard sputtering process. In this embodiment,the carbon of coating layer 30 was applied in a uniform manner toachieve a thickness of about 500 Å. Coating layer 30 of this particularembodiment was also shown to be effective at reducing the driftexhibited by the prior art, as best shown with reference to FIG. 4. Asbefore, trace 31 represents the relative offset of the reflectedwavelength of pressure grating 12 and trace 32 represents the relativeoffset of the reflected wavelength of temperature grating 13 as measuredover about a five-day testing period in conditions substantiallyidentical to those described herein above referring to FIG. 2. (The testresults in FIG. 4 were terminated earlier than in FIG. 2 because of theclose correlation of the results. However, subsequent long term testinghas validated the robustness of the coating.) The reflected wavelengthof pressure grating 12 (trace 31) shows a fairly stable averagereflected relative wavelength of about 1.0072 nanometers for the entireduration of the test with no perceptible shift due to water ingression.Similarly, the reflected wavelength of temperature grating 13 (trace 32)shows an almost imperceptible change over the same time period. Thecarbon coating 30 of this embodiment thus represents an improvementwhich is at least twenty times as effective at blocking water, and itsdeleterious effects, when compared to the prior art.

In an alternative embodiment of the present invention, the shell 20 anda portion of or all of the tube-encased fiber grating 1 may be replacedby a large diameter silica waveguide grating, such as that described incopending U.S. patent application Ser. No. 09/455,868, entitled “LargeDiameter Optical Waveguide, Grating and Laser,” which is incorporatedherein by reference. The waveguide includes coating 30 as describedhereinabove to provide fluid blocking capability in accordance with thepresent invention.

As stated before, any method of coating at least shell 20 of transducer1 with a fluid blocking coating 30 is contemplated by the presentinvention. Coating 30 may be applied to shell 20 after the shell hasbeen disposed about the fiber 10, and capillary tube 15 (if applicable),but may be applied earlier in the process without departing from thescope of the present invention. One known method of providing coating 30comprises the sputtering of the coating onto the glass shell 20. Priorto the sputtering process, shell 20 is prepared to ensure that coating30 makes intimate contact with the surface of the shell. In oneembodiment, shell 20 is prepared for coating by wiping the outsidesurface of the shell, as well as other outside surfaces to be coated,such as capillary tube 15 and fiber 10 if applicable, with a degreasingsolution, such as acetone. The surface may then be etched to enhance theadhesion of the coating to the shell. In one embodiment, shell 20 issubjected first to an oxygen-ion etch followed by an argon-ion etch.Subsequent to such etching, coating 30 is deposited onto the outsidesurface by sputtering or other similar coating processes that ensureuniform coverage of the shell (and other components).

It should be understood that the dimensions, geometries, and materialsdescribed for any of the embodiments herein are merely for illustrativepurposes and as such any other dimensions, geometries, or materials maybe used if desired, depending on the application, size, performance,manufacturing or design requirements, or other factors, in view of theteachings herein.

For instance, the present invention further comprises a fluid blockingfiber optic pressure sensor, wherein the optical sensing element and theshell are comprised of the same material and essentially constitute arelatively large diameter fiber section. In this particular embodiment,at least a portion of the sensing element has a transverse cross-sectionwhich is contiguous, is made of substantially the same material, whichhas an outer transverse dimension of at least 0.3 mm, and which has afluid blocking coating disposed on the external surface of the sensingelement.

Further, it should be understood that, unless otherwise stated herein,any of the features, characteristics, alternatives or modificationsdescribed regarding a particular embodiment herein may also be applied,used, or incorporated with any other embodiment described herein. Also,the drawings shown herein are not drawn to scale.

Although the invention has been described and illustrated with respectto exemplary embodiments thereof, the foregoing and various otheradditions and omissions may be made thereto without departing from thespirit and scope of the present invention.

1. A pressure sensor for detecting an external pressure, comprising: anoptical waveguide, wherein the waveguide comprises an optical pressuresensor; a unitary quartz shell encasing the optical waveguide and fortransmitting the external pressure to the optical pressure sensor; and anon-polymeric coating formed on at least an external surface of theshell to prevent moisture ingress from affecting the optical pressuresensor.
 2. The pressure sensor of claim 1, wherein the shell is coupledto at least a portion of the optical waveguide.
 3. The pressure sensorof claim 1, wherein the shell comprises a tube.
 4. The pressure sensorof claim 1, wherein the coating comprises at least one layer.
 5. Thepressure sensor of claim 1, wherein the coating is selected from thegroup consisting of gold, chrome, silver, carbon or silicon nitride. 6.The pressure sensor of claim 1, wherein the coating is capable ofpreventing the diffusion of water molecules into the shell.
 7. Thepressure sensor of claim 1, wherein the coating comprises: a first layercomprising chrome formed on the external surface of the shell; and asecond layer comprising gold formed on the first layer.
 8. The pressuresensor of claim 7, wherein the first layer has a thickness of about 250Å and wherein the second layer has a thickness of about 20,000 Å.
 9. Thepressure sensor of claim 1, wherein the optical pressure sensor is aFiber Bragg Grating.
 10. The pressure sensor of claim 1, wherein theoptical waveguide further comprises an optical temperature sensor. 11.The pressure sensor of claim 10, wherein the optical temperature sensorand the optical pressure sensor are connected along the opticalwaveguide in series.
 12. The pressure sensor of claim 1, furtherincluding a capillary tube positioned between the shell and the opticalwaveguide.
 13. The pressure sensor of claim 12, wherein the capillarytube contains a narrowed portion, and wherein the optical pressuresensor is located in the narrowed portion.
 14. The pressure sensor ofclaim 1, wherein the shell is comprised of a glass.
 15. The pressuresensor of claim 1, wherein the shell has an outside diameter of 0.3 mm.16. The pressure sensor of claim 15, wherein the shell and the opticalwaveguide are contiguous and are made of the same material.
 17. Thepressure sensor of claim 1, wherein the shell is capable of beingstrained due to a change in the external pressure, such strain causing achange in an optical characteristic of the optical pressure sensorindicative of the external pressure.
 18. The pressure sensor of claim17, further comprising a temperature sensor in the optical waveguide,wherein the temperature sensor is positioned so that it is notsubstantially affected by the change in the external pressure.
 19. Thepressure sensor of claim 1, wherein the optical waveguide comprises anarrow diameter portion between two thick portions thicker than thenarrow diameter portion, and wherein the optical pressure sensor islocated in the narrow diameter portion.
 20. The pressure sensor of claim19, wherein the shell is affixed to the two thick portions.
 21. A methodfor forming a pressure sensor for detecting an external pressure,comprising: providing an optical waveguide, wherein the waveguidecomprises an optical pressure sensor; encasing the optical waveguide ina unitary quartz shell, wherein the shell is capable of transmitting theexternal pressure to the optical pressure sensor; and forming anon-polymeric coating on at least an external surface of the shell toprevent moisture ingress from affecting the optical pressure sensor. 22.The method of claim 21, further comprising coupling the shell to atleast a portion of the optical waveguide prior to the coating step. 23.The method of claim 21, wherein the shell comprises a tube.
 24. Themethod of claim 21, wherein the coating comprises at least one layer.25. The method of claim 21, wherein the coating is selected from thegroup consisting of gold, chrome, silver, carbon or silicon nitride. 26.The method of claim 21, wherein the coating is capable of preventingdiffusion of water molecules into the shell.
 27. The method of claim 21,wherein forming the coating comprises: forming a first layer comprisingchrome on the external surface of the shell; and forming a second layercomprising gold on the first layer.
 28. The method of claim 27, whereinthe first layer is formed to a thickness of about 250 Å and wherein thesecond layer is formed to a thickness of about 20,000 Å.
 29. The methodof claim 21, wherein the optical pressure sensor is a Fiber BraggGrating.
 30. The method of claim 21, wherein the optical waveguidefurther comprises an optical temperature sensor.
 31. The method of claim30, wherein the optical temperature sensor and the optical pressuresensor are connected along the optical waveguide in series.
 32. Themethod of claim 21, further comprising forming a capillary tube betweenthe shell and the optical waveguide.
 33. The method of claim 32, whereinthe capillary tube contains a narrowed portion, and wherein the opticalpressure sensor is located in the narrowed portion.
 34. The method ofclaim 21 wherein the shell is comprised of a glass.
 35. The method ofclaim 21, wherein the shell has an outside diameter of 0.3 mm.
 36. Themethod of claim 35, wherein the shell and the optical waveguide arecontiguous and are made of the same material.
 37. The method of claim21, wherein the shell is capable of being strained due to a change inthe external pressure, such strain causing a change in an opticalcharacteristic of the optical pressure sensor indicative of the externalpressure.
 38. The method of claim 21, wherein forming the coatingcomprises a sputtering process.
 39. The method of claim 21, furthercomprising preparing the external surface prior to forming the coating.40. The method of claim 39, wherein the preparing comprises degreasingthe external surface, or etching the external surface, or both.
 41. Themethod of claim 40, wherein etching comprises an ion etching process.42. The method of claim 21, wherein the shell is fused to the opticalwaveguide.
 43. The method of claim 21, wherein encasing the opticalwaveguide in a shell comprises fusing the shell to the opticalwaveguide.
 44. The method of claim 21, wherein the optical waveguidecomprises a narrow diameter portion between two thick portions thickerthan the narrow diameter portion, and wherein the optical pressuresensor is located in the narrow diameter portion.
 45. The method ofclaim 44, wherein the shell is affixed to the two thick portions.